Calcium-ion binding to the chromaffin-granule surface

Abstract

1. Ca²⁺ions decreased the surface charge of isolated adrenal-medullary chromaffin granules whether the granules were untreated or previously incubated with neuraminidase. 2. Ca²⁺binding in both cases followed a Langmuir adsorption isotherm. 3. The chromaffin-granule surface was essentially polyanionic with about 10800 anionic sites per granule, of which 4200 sites are capable of binding Ca²⁺ions with an electrochemical free energy of adsorption of −1.32kJ·mol⁻¹. 4. The surface region of the chromaffin granule was calculated to bind 1306 Ca²⁺ions at 2.2mm-Ca²⁺(ionic strength 0.16mol·litre⁻¹). 5. The importance of Ca²⁺binding to the chromaffin-granule surface is discussed in relation to the hypothesis of secretion by exocytosis.